Le Rôle des Complexes A Liaisons Hydrogène “Homéotopiques” Dans Les échanges Rapides Entre Protons

In some hydrogen-bonded complexes, the bonding proton can occupy two positions, such as the motion from the first to the latter does not modify the general aspect of the complex. To this class belong, for instance, the ammonium-amine complexes, the four-centers transition complexes between OH and NH groups discussed by Brodskii, the cyclic dimers of carboxylic acids and so on. Quite generally the presence of such complexes with “homeotopous” bonds leads to a very fast exchange of the protons. The exchange rate is given by νexch=k [complex] where k is the rate constant for the rupture of the hydrogen bond, with an order of magnitude of 109 sec−1 and [complex], the equilibrium concentration of the complex obeys, in first approximation to the law of mass-action. The knowledge of the equilibrium constant can given information about the exchange rate. For the exchange between amines and alcohols, for instance, one can deduct from our experimental complexation constants, caracteristic exchange-times of 10−8 to 10−7 sec. Alternatively one can predict the order of magnitude of complexation constants from experimental values of the exchange rate. From the rate constants of proton exchange between methyl ammonium salts and water, determined by Grunwald, Loewenstein, Meiboom and coworkers, one can deduct a order of magnitude of 0,1 to 1 M−1 for the complexation constants between ammonium ions and amine. On the other hand, the exchange rates through such complexes appear to be little temperature dependent because the complexation energy acts in opposite sense on k and on the complexation constant. The only term that remains is the activation energy for the formation of the H-bond, that may be very small. Experimental results of Conner and Loewenstein seem fit with this prediction.